In a number of processes, such as the refining of crude oil, the purification of natural gas and the production of synthesis gas from, for example, fossil fuels, sulphur containing gas, in particular H2S containing gas, is released. On account of its high toxicity and its smell, the emission of H2S is not permissible.
The best known and most suitable process for removing sulphur from gas by recovering sulphur from hydrogen sulphide is the so-called Claus process. In this process hydrogen sulphide is converted by oxidation to a considerable extent into elemental sulphur; the sulphur thus obtained is separated from the gas by condensation. The residual gas stream (the so-called Claus residual gas) still contains some H2S and SO2.
The method of recovering sulphur from sulphur containing gases by the so-called Claus process is based on the following overall reactions:
2 H2S+3 O2xe2x86x922 H2O+2 SO2xe2x80x83xe2x80x83(1)
4 H2S+2 SO2⇄4 H2O,+6/n Snxe2x80x83xe2x80x83(2)
Reactions (1) and (2) result in the main reaction:
2 H2S+O2⇄2 H2O+2/n Snxe2x80x83xe2x80x83(3)
A conventional Claus converterxe2x80x94suitable for processing gases having an H2S content of between 50 and 100%, comprises a burner with a combustion chamber, the so-called thermal stage, followed by a number of reactors generally two or threexe2x80x94filled with a catalyst. These last stages constitute the so-called catalytic stages.
In the combustion chamber, the incoming gas stream, which is rich in H2S, is combusted with an amount of air at a temperature of approximately 1200xc2x0 C. The amount of air is adjusted so that one third of the H2S is fully combusted to form SO2 in accordance with the following reaction:
xe2x80x832 H2S+3 O2xe2x86x922 H2O+2 SO2xe2x80x83xe2x80x83(1)
After this partial oxidation of H2S the non-oxidised part of the H2S (i.e. basically two-thirds of the amount offered) and the SO2 formed react further as to a considerable portion, in accordance with the Claus reaction
4 H2S+2 SO2⇄4 H2O+3 S2xe2x80x83xe2x80x83(2)
Thus, in the thermal stage, approximately 60% of the H2S is converted into elemental sulphur.
The gases coming from the combustion chamber are cooled to about 160xc2x0 C. in a sulphur condenser, in which the sulphur formed is condensed, which subsequently flows into a sulphur pit through a siphon.
The non-condensed gases, in which the molar ratio of H2S:SO2 is unchanged and still 2:1, are subsequently heated to about 250xc2x0 C., and passed through a first catalytic reactor in which the equilibrium
4 H2S+2 SO2⇄4 H2O+6/n Snxe2x80x83xe2x80x83(2)
is established.
The gases coming from this catalytic reactor are subsequently cooled again in a sulphur condenser, in which the liquid sulphur formed is recovered and the remaining gases, after being re-heated, are passed to a second catalytic reactor.
When the gaseous feedstock contains H2S concentrations of between about 15 and 50%, the above described xe2x80x9cstraight throughxe2x80x9d process is not used, but instead a variant thereof, the so-called xe2x80x9csplit-flowxe2x80x9d process. In the latter process one-third of the total amount of feedstock is passed to the thermal stage and combusted completely to SO2 therein. Two-thirds of the feedstock is passed directly to the first catalytic reactor, by-passing the thermal stage. When the feedstock contains H2S concentrations of between 0 and 15 the Claus process can no longer be used. The process then used is, for example, the so-called Recycle SELECTOX process, in which the feedstock is passed with an adjusted amount of air into an oxidation reactor, the so-called oxidation reactor, the so-called oxidation stage. The reactor contains a catalyst which promotes the oxidation of H2S to SO2, and the amount of oxidation air is adjusted so that an H2S:SO2 ratio of 2:1 is established, whereafter the Claus reaction proceeds. The gas from the oxidation reactor is cooled in a sulphur condenser, in which the sulphur formed is condensed and discharged.
To dissipate the reaction heat generated in the oxidation reactor, a portion of the gas stream coming from the sulphur condenser is recirculated to the oxidation reactor.
It is clear that in the Recycle SELECTOX process, the oxidation stage, which is catalytic and does not lead to high temperatures, is equivalent to the thermal stage in the Claus process. In the following, both the thermal Claus stage and the oxidation stage of the Recycle SELECTOX process are referred to as oxidation stages.
The sulphur recovery percentage in a conventional Claus converter is 92-97%, depending on the number of catalytic stages
By known processes, the H2S present in the residual gas from the Claus reaction is converted, by combustion or some other form of oxidation, into SO2 whereafter this SO2 is emitted to the atmosphere. This has been permissible for low concentrations or small amounts of emitted SO2 for a long time. Although SO2 is much less harmful and dangerous than H2S this substance is also so harmful that its emission is also limited by ever stricter environmental legislation.
As has been observed, in the Claus process as described above, in view of the equilibrium reaction which occurs, the H2S:SO2 ratio plays an important role. In order to obtain an optimum conversion to sulphur, this ratio should be 2:1. Generally speaking, this ratio is controlled by means of a so-called H2S/SO2 residual gas analyser. This analyser measures the H2S and SO2 concentrations in the residual gas. A controller then maintains the ratio of 2:1 constant on the basis of the equation
xe2x80x83[H2S]xe2x88x922[SO2]=0
by varying the amount of combustion air, depending on the fluctuations in the gas composition and the resulting deviation in the above equation. Such a control of the process, however, is highly sensitive to these fluctuations.
Furthermore, the sulphur recovery efficiency (calculated on the amount of H2S supplied) is no higher than 97%, and so the gas flowing from the last catalytic stage the residual gasxe2x80x94still contains substantial amounts of H2S and SO2, determined by the Claus equilibrium, and this in a molar ratio of 2:1.
The amount of H2S present in the residual gas can be separated by absorption in a liquid.
The presence of SO2 in the residual gas, however, is a disturbing factor during the further processing thereof and must therefore be removed prior to such further processing. This removal and hence the after-treatment of the gas is complicated.
The great disadvantage of the presence of SO2 is that this gas reacts with conventional liquid absorbents to form undesirable products. To prevent undesirable reactions of the SO2, therefore, the SO2 is generally catalytically reduced with hydrogen to form H2S over an Al2O3 supported cobalt-molybdenum catalyst in accordance with the so-called SCOT process. The total amount of H2S is subsequently separated by liquid absorption in the usual manner.
In the SCOT process the sulphur components, other than H2S, such as SO2 (sulphur dioxide) and sulphur vapour (S6 and S8) are fully hydrogenated to H2S according to the following reactions:
SO2+3 H2xe2x86x92H2S+2 H2Oxe2x80x83xe2x80x83(4)
S6+6 H2xe2x86x926 H2Sxe2x80x83xe2x80x83(5)
S8+8 H2xe2x86x928 H2Sxe2x80x83xe2x80x83(6)
Other components, such as CO, COS and CS2, are hydrolysed according to:
xe2x80x83COS+H2Oxe2x86x92H2S+CO2xe2x80x83xe2x80x83(7)
CS2+2 H2Oxe2x86x922 H2S+CO2xe2x80x83xe2x80x83(8)
CO+H2Oxe2x86x92H2+CO2xe2x80x83xe2x80x83(9)
Above conversions to H2S are performed with a cobalt-molybdenum catalyst on alumina at a temperature of about 280-330xc2x0 C. For the SCOT process it is required that sulphur vapour is hydrogenated to H2S, and also that SO2 is completely converted to H2S down to ppm level, to prevent plugging/corrosion in the down-stream water quench column. This type of hydrogenation can be defined as high temperature hydrogenation.
In accordance with another method, for example, the BSR SELECTOX process, after reduction of the SO2 in residual gas to H2S and after condensation of the water vapour, the gas is passed into an oxidation reactor, as in the Recycle SELECTOX process. The oxidation air is adjusted so that an H2S:SO2 ratio of 2:1 is obtained, whereafter the Claus reaction proceeds. Both in the SCOT process and in the BSR SELECTOX process, the removal of SO2 from the residual gas is a relatively expensive operation.
The above described after-treatment of the gases, carried out by means of a so-called tail gas treater, which involves an investment of another 50-100% of the cost of the preceding Claus converter, can result in an increase of the sulphur recovery efficiency of up to 98-99.8%.
In U.S. Pat. No. 4,988,494, it is proposed that the H2S concentration in the gas leaving the last catalytic Claus stage is controlled to have a value ranging between 0.8 and 3% by volume by reducing the quantity of combustion or oxidation air passed to the oxidation stage.
The increase of the H2S concentration will result in a decreased SO2 concentration, however, not to very low levels. For an H2S concentration of 0.8% by volume, the SO2 concentration will be typically 0.03-0.15% by volume, and this will result in a sulphur recovery efficiency loss of typically 0.09-0.45%.
In the process according to this patent, the H2S is selectively oxidised in a dry bed oxidation stage.
As SO2 is not converted in a dry-bed oxidation stage, this will result in appreciable sulphur recovery losses, and consequently sulphur recovery efficiencies close to 100% can not be reached.
A second disadvantage of operating with excess H2S compared to SO2 is that the temperature increase in the drybed oxidation reactor becomes higher with increasing H2S concentration.
Higher reactor temperatures will result in an increased formation of SO2 as a result of gas-phase and catalytic oxidation of formed sulphur vapour. Also for this reason, a shifted operation towards H2S of the Claus converter is not beneficial.
It has been experienced, that in case the catalyst bottom temperature in a dry-bed oxidation reactor exceeds 250-260xc2x0 C., the oxidation efficiency to elemental sulphur will start to drop from 94-96% to lower values. Combined with a reactor inlet temperature of approximately 180-200xc2x0 C., this results in a temperature increase of some 60-80xc2x0 C., corresponding with 1.0-1.2 vol. % of H2S in the process gas.
The shifted operated sulphur plant, followed by a drybed oxidation step with an oxidation catalyst which is not effective in promoting the Claus reaction, is known as the SUPERCLAUS(trademark) or SUPERCLAUS(trademark)-99 process.
The SUPERCLAUS(trademark) process, as well as the SUPERCLAUS(trademark)-99.5 process, is described in -SUPERCLAUS(trademark)-the answer to Claus plant limitationsxe2x80x3, Lagas, J. A.; Borsboom, J; Berben, P. H., 38th Canadian Chemical Engineering Conference, Edmonton, Canada.
It is known, that removal of SO2 from a process gas can be performed according to a high temperature hydrogenation step to H2S as applied in the SCOT process or in the SUPERCLAUS(trademark)-99.5 process.
In European patent No. 669,854 the selective hydrogenation of SO2 to elemental sulphur has been described. It has been mentioned that this hydrogenation could suitably be applied downstream of a regular Claus unit, prior to dry bed oxidation.
However, the specific process conditions required according to this patent are not easily compatible with the composition of gas coming from a Claus unit. This means that rather complicated, and thus costly, measures are necessary to satisfy these conditions.
Surprisingly it has now been found that it is possible to increase the sulphur recovery by very simple process modifications.
According to the invention a process for the recovery of sulphur from a hydrogen sulphide containing gas comprises:
i) oxidising part of the hydrogen sulphide in a gaseous stream with oxygen or an oxygen containing gas in an oxidation stage to sulphur dioxide;
ii) reacting the product gas of this oxidation stage in at least two catalytic stages, in accordance with the Claus equation
2 H2S+SO2xe2x86x922 H2O+3/n Sn
iii) catalytically reducing SO2 in the gas leaving the last of said at least two catalytic stages, wherein the catalytic reduction takes place in a catalyst bed downstream from the last Claus catalytic stage.
One aspect of the invention is the reduction of SO2. This reduction of SO2 to elemental sulphur, to H2S or to a mixture of both, by hydrogenation, requires the presence of hydrogen. In the gas leaving the last catalytic Claus stage sufficient hydrogen is normally present. This hydrogen is produced in the thermal stage, by, among others, cracking of H2S to hydrogen and sulphur vapour. In case the amount of hydrogen is insufficient, additional hydrogen may be added to the gas, by adding a stream of concentrated hydrogen, or by generating hydrogen by sub-stoichiometric combustion of fuel gas in in-line process heaters.
Also CO is usually present in the gas containing SO2. Because of the reducing properties of CO, this component is capable of reducing SO2. In this way CO acts in the same way as hydrogen, and a mixture of hydrogen and CO is therefor also suitable for reducing SO2.
CO+H2Oxe2x86x92CO2+H2
SO2+2 COxe2x86x922CO2+1/n Sn